A heat pump water heater system potentially has a much lower operating cost than a system using electric resistance heating or one in which high priced fossil fuel is burned. Since a heat pump can extract heat energy from a water source or form outdoor ambient air, it may use as little as 1/3 the electrical energy required by electrical resistance heating. However, the efficiency of a typical heat pump system for heating either water or air drops substantially as the temperature of the heat source falls below the optimum range for which the system is designed. For example, a conventional single stage heat pump system using refrigerant fluid R-22 has an efficiency less than that of electric resistance heating, if the source temperature falls below approximately 20.degree. F.
Although refrigerants other than R-22 may be used in a heat pump water heater system to extract energy from a heat source of much lower temperature, such systems do not produce very hot water, nor operate efficiently at higher source temperatures. For applications in which a heat pump must extract heat from a source of variable temperature, it is common practice to design the system for relatively higher source temperatures and to provide backup or auxiliary heat in the form of electric resistance elements or fossil fuel burners, for use when the source temperature drops below an economic balance point.
An alternative solution to this problem is disclosed in U.S. Pat. No. 3,301,002 wherein two compressors are staged and operated as a primary and secondary refrigerant loop. The secondary system may be operated to heat water while the primary system is inoperative, or both may be operated to temperature condition air supplied to a comfort zone and to heat water, over a wide range of ambient air temperatures.
A system capable of extracting heat from relatively low temperature ambient air (-10.degree. C.) to produce hot water (up to 80.degree. C.) is disclosed in U.S. Pat. No. 3,984,050. This system includes two compressors which are cascade coupled, with the evaporator of one serving to condense refrigerant fluid from the other. Water heated in the system is supplied both to radiators disposed to heat a plurality of comfort zones, and to a potable water heater.
An economical water heating system is particularly important in commercial applications such as hospitals and restaurants which often require significant quantities of hot water for use in laundry or washing facilities. A heat pump water heating system is a logical choice for such applications, however, the system should be capable of operating over a wide range of source temperature. Since the initial cost of such a system is significant compared to other types of water heaters, it should also provide supplemental comfort zone temperature conditioning, further reducing its pay-back period.
In consideration of the foregoing, it is an object of this invention to provide a heat pump system capable of heating water using heat extracted from a source of variable temperature.
It is a further object of this invention to provide supplemental heating and cooling in a comfort zone.
A still further object is to provide common means for heating or cooling air supplied to a comfort zone and for transferring heat between a first and a second refrigerant fluid in a cascade heat pump water heater system.
These and other objects of the invention will be apparent from the description of the preferred embodiment of the invention which follows and from the attached drawings.